1. Field of the Invention
This invention relates to audio signal transmission systems and, more particularly, to the use of carrier frequency shift supervisory signaling in such systems.
2. Description of the Prior Art
In many voice signal transmission systems, and particularly in cordless telephone station systems, it is necessary to transmit a plurality of supervisory states as well as voice signals. More specifically, a cordless transmitter in such a system must be battery operated, and it is desirable to provide some type of alarm to indicate a low battery condition. More generally, supervisory states such as OFF and ON must likewise be transmitted from the portable transmitter.
It is well known to use frequency modulation to impress voice signals on a carrier signal in a transmission system. It is also known to utilize frequency shift techniques to transmit binary states in a transmission system. It has been difficult, however, to combine frequency modulation transmission of audio signals with frequency shift techniques for transmitting supervisory states. Moreover, when a plurality of supervisory states must be transmitted, the problem becomes further compounded.